Conventionally, a light source device including a semiconductor light-emitting element, a phosphor, and a wavelength selection filter has been known. The semiconductor light-emitting element is a light source that excites the phosphor. The semiconductor light-emitting element is, for example, a light emitting diode (LED), a laser diode (LD), or the like. For example, patent reference 1 proposes a vehicle lamp device capable of improving the light utilization efficiency by using a phosphor and a wavelength selection filter in the vehicle lamp device.
According to the technique described in patent reference 1, the vehicle lamp device irradiates the phosphor (a phosphor element) with blue light (excitation light), thereby obtaining yellow light (converted light) isotropically radiated from the phosphor. The converted light is fluorescence light emitted by the phosphor. The yellow light radiated from the phosphor in a direction opposite to an optical path traveling direction is reflected by the wavelength selection filter (a wavelength selection member) in the optical path traveling direction. In this way, the light utilization efficiency can be improved. In addition, the phosphor has a property of transmitting part of the blue light (the excitation light) instead of converting all the blue light (the excitation light) into the yellow light (the converted light). Thus, white light which is a mixture of the yellow light (the converted light) of the yellow wavelength and the blue light (the excitation light) of the blue wavelength can be obtained.
The “optical path traveling direction” is a direction in which the light emitted from the light source device, the vehicle lamp device, or the like travels. Namely, the “optical path traveling direction” is a light traveling direction on the optical path.